In a long term evolution-advanced (LTE-A) system, after a carrier aggregation (CA) technology is applied, user equipment can access multiple component carriers at the same time to transmit and receive data. To support hybrid automatic retransmission, the user equipment needs to feed back a hybrid automatic repeat request-acknowledgement (HARQ-ACK) response to a base station by using an uplink channel. In an existing CA system, the HARQ-ACK response is sent only on a primary component carrier.
In subsequent evolution systems, carrier aggregation evolves to be aggregation across different duplexing manners, and/or across different base stations. In this scenario, for user equipment that has only an uplink single transmission capability, in one subframe, the user equipment can perform uplink transmission in only one serving cell, and therefore, the user equipment needs to operate across different serving cells in a time division multiplexing manner. Therefore, for each serving cell, in one radio frame, only some subframes are used for uplink transmission. For an FDD serving cell, according to existing HARQ-ACK timing of the FDD serving cell, because in one radio frame, only some uplink subframes can be used for uplink transmission of the serving cell, some downlink subframes in one radio frame do not have corresponding uplink subframes for feeding back HARQ-ACK responses. Consequently, these downlink subframes cannot be invoked, and the utilization of system resources is significantly decreased.